1. Field of the Invention
The present invention concerns a magnetic resonance antenna of the type having longitudinal antenna rods disposed in parallel to each other in a birdcage structure, the longitudinal antenna rods being connected in terms of radio-frequency, to a ferrule or ring at each end, and radio-frequency switching elements connected to interrupt. In terms of radio-frequency at least one part of the longitudinal antenna rods to detune the eigen-resonance frequency of the antenna with regard to a working (operating) magnetic resonance frequency.
2. Description of the Prior Art
Modern magnetic resonance systems normally operate with a number of different antennas (also called coils in the following) to emit radio-frequency pulses for magnetic resonance excitation and/or to receive the resulting magnetic resonance signals. A magnetic resonance system typically has a larger coil, known as a whole-body coil, also called a bodycoil (BC), that normally is integrated into the device, as well as a number of small local coils (LC), also called surface coils. In contrast to the whole-body coil, the local coils (LC) serve to acquire detailed images of body parts or organs of a patient that are located relatively close to the surface of the body. For this purpose, the local coils are directly applied to the location of the patient at which the region to be examined is located. Given the use of such a local coil, in many cases magnetic resonance signals are excited with the whole-body coil (as a transmitter coil) integrated into the magnetic resonance system and are received with the local coil (as a receiver coil). So that the coils do not interact with one another, it is necessary to detune the receiver coil in the transmission phase and the transmitter coil in the reception phase. For detuning, the eigen-resonance frequency of the respective antenna is adjusted (set) such that it no longer lies in the range of the working magnetic resonance frequency. In the ideal case, an antenna detuned in this manner behaves neutrally. i.e. it is transparent to the radio-frequency energy emitted by the other coil and to the magnetic resonance signals induced by the other coil. If switching between two different antennas continuously occurs, this transitional detuning in the transmission phase or in the reception phase is called a “dynamic detuning”. A coil also can be permanently detuned, however, if it should only be operated with one other coil. Such a “static detuning” is necessary particularly in cases when a transmission-capable local coil is used that assumes both the transmission and the reception function. Since the larger whole-body coil integrated into the system cannot be physically removed during the imaging measurement, it is electrically deactivated by the detuning.
A number of magnetic resonance antennas having a structure known as a birdcage structure are used as a whole-body coil. Such an antenna has a number of longitudinal, parallel antenna rods defining a cylindrical surface that are connected in terms of radio-frequency with one another at their ends by antenna ferrules or rings. The longitudinal antenna rods and antenna ferrules in principle can be fashioned in an arbitrary forms (shapes). In many cases they are conductor runs (tracks) that are applied to a flexible insulating foil and are cylindrically wound around the measurement space in which the examination subject is located during the examination. In a whole-body coil, the birdcage structure surrounds the patient acceptance chamber in which the patient is positioned during the measurement. In local coils having the form of a birdcage structure, the measurement volume of the local coil serves for exposure of the head of other extremities of a patient in order to precisely examine this region.
In principle there are various possibilities to detune such magnetic resonance antennas having a birdcage structure.
If the field strength of the basic magnetic field (also called B0 field in the following) of the magnetic resonance system is below two Tesla, detuning is likely possible via the radio-frequency feed (supply line). To detune the antenna by means of a suitable circuit element, for example a PIN diode or a relay, a short circuit is produced at the end of the feed farthest from the coil. This short is transmitted via the feed to the feed point, i.e. the connection point, at which the feed is connected to the antenna. The detuning thereby achieved is sufficient to suppress coupling with the other active antenna. The advantage of such a coil-removed detuning is that the supply line for the direct current necessary for the circuit element presents no problem since, due to the large distance, no interaction of the circuit direct current with the high static and radiation detector fields is to be expected in the immediate environment of the antenna.
Such coil-removed detuning, however, has not proven to be suitable given higher B0 field strengths. In such cases, ft is necessary to install the detuning elements directly into the structure of the antenna. For detuning an antenna with a birdcage structure, this can ensue by detuning that either the antenna ferrules, the longitudinal antenna rods or both. In such detuning, in general a resonant inductance is interrupted by means of a suitable radio-frequency switching element, or a resonant capacitor is bridged, i.e. shorted. Currently switching diodes, for example PIN diodes, are used as such radio-frequency switching elements, since these are able to withstand in terms of radio-frequency, both high currents and high voltages, and in addition can be switched sufficiently rapidly.
Ferrule detuning in a birdcage structure (compared to rod detuning) is advantageous because of easier access to the detuning elements, meaning the RF switching elements, from the exterior, such that the necessary direct current supply lines can be installed without problems. In terms of radio-frequency, however, such a ferrule detuning is not the optimal solution in a birdcage structure. In practice, antennas with birdcage structures therefore are known in which radio-frequency switching elements with which the longitudinal rods can be interrupted, so the entire rod structure can be detuned, are arranged within the longitudinal rods (meaning the longitudinal inductances). Each such RF switching element is individually fed with the necessary direct current signals from the outside, i.e. from outside the birdcage structure, A significant disadvantage of this approach is that these supply lines must be run through the resonant structure without disturbing the antenna with regard to its radio-frequency function. Each individual direct current supply line therefore must be individually choked and decoupled, for which special, very complexly manufactured lines must be used. Aside from the fact that space relationships inside the resonant structure are crowded and accessible components for the choking and decoupling can be accommodated only with difficulty, this assembly is very costly due to its high production complexity.
A birdcage antenna structure is known from published Japanese Application 08 187 235 in which each longitudinal rod is interrupted by two switching diodes connected with opposite polarities with regard to the longitudinal rod, but connected rectified with one another. The switching diodes of two adjacent longitudinal rods are in turn respectively connected in the same direction via LC parallel resonant circuits. Overall, a radio-frequency-damped ring switching line is formed in which all switching diodes are serially switched in succession. All switching diodes therefore can be switched together via supply lines directed out of the birdcage structure at both ends of the ring switching line. This assembly has the disadvantage, however, that a relatively high switching voltage must be applied to the switching line to detune the antenna, since the necessary blocking voltage (off-state voltage) applied at the individual switching diodes is determined by dividing the applied switching voltage by the total number of diodes. Moreover, the switching line must be relatively well-shielded, otherwise the magnetic field emitted by the switching line upon the rapid switching events will disturb the basic magnetic field of the magnetic resonance system. A further disadvantage is that the failure of an individual switching diode can block the entire function of the switching line, thus the assembly is unavoidably fault-prone.